Cytotoxic T lymphocytes (CTL) are white blood cells found in the blood, spleen and lymph. CTL have the ability to attack and kill other cells of the body in a highly specific manner. When CTL are stimulated by specific antigen, they migrate through the tissues of the body on a “search and destroy” mission for cells bearing the specific antigen. Whether of viral origin or tumor associated, CTL detect antigen that is bound to major histocompatability complexes (MHC) on the surface of potential target cells. Once CTL have identified the antigen on the cell surface, their function is to deliver a lethal hit to the cell.
Although there are hundreds of millions of CTL that reside in the spleen, each individual CTL exclusively responds to a unique and specific antigen. These individual CTL, dubbed CTL precursors (CTLp), undergo cell division or proliferate upon activation by specific antigen to produce daughter cells with precisely the same antigen specificity as the parent cell. This proliferation increases the total number, and thus the frequency, of that specific CTLp in the body. A proportion of these newly generated CTL briefly recirculate through the body (termed effector CTL, and have the ability to identify and destroy cells bearing the specific antigen which they recognize. A significant body of experimental evidence suggests that CTL specific for tumor antigens can inhibit tumor growth. Unfortunately, most tumors have only a very weak capacity to stimulate CTL responses and there has been no means of inducing a CTL response then sustaining it over a period of time sufficient to continuously inhibit tumor growth. While many attempts to directly increase the capacity of tumor cells to stimulate tumor-clearing CTL responses in patients have been made, such attempts have met with limited success. Technical advances over the past ten years have, however, enabled the identification of natural peptide antigens that are present on tumor cells and which are recognized by CTL. These antigen targets include proteins expressed in significant overabundance, abnormally expressed embryonic proteins, protein products from mutated oncogenes or suppressor genes, or proteins derived from cancer-causing viruses present in tumor cells. The challenge has been to find a way in which to administer an antigen so that it induces an anti-tumor CTL response and maintains it over time. While many attempts have now been made to use these antigens clinically in a vaccine, the results have been less than satisfactory.
An explanation of why CTL therapies have been largely ineffective at eradicating or controlling tumors in a clinical setting include the following:                (a) Vaccine designs have been inadequate at initiating strong CTL responses;        (b) Tumor cells can down regulate MHC molecules, resulting in the loss of antigen presentation from the surface of cells, thereby escaping detection by CTL;        (c) After induction, effector CTL recirculation through the body is highly transient;        (d) After recirculation, CTL return to the spleen where they reside in a non-active or resting state, and an increase in the numbers of CTLp residing in the spleen does not reflect active CTL immunity;        (e) In the case of tumors, regrowth of residual tumor cells following immunization goes undetected by CTLp residing in spleen in a “resting” state;        (f) Because CTL-stimulating antigen-presenting cells (APC) are targeted for destruction by the same CTL that they have activated, the CTL response is self-limiting, which precludes, under normal circumstances, the continuous stimulation for a long-lived CTL response.        
A growing repertoire of tumor associated antigens are being discovered that are recognized by CTL. A variety of techniques have been suggested to render these antigens effective in CTL vaccines. These include immunization using synthetic peptide antigens mixed with an immunostimulatory adjuvant, such as the bacterial toxin BCG; immunization with multiple antigenic peptide systems (MAPS); immunization with “professional” antigen presenting cells, which are isolated from the patient, pulsed with peptide antigen and inoculated back into the patient as a vaccine; immunization with peptides designed to stimulate both CTL and T helper cell populations; immunization with viruses or bacteria engineered to express tumor antigens; and immunization with polynucleotide expression vectors (so called DNA vaccines). Unfortunately, none of these approaches has been an unqualified success. As discussed above, the lack of vigorous therapeutic effects with these vaccine platforms reflects at least to some degree problems associated with inducing a strong initial CTL response and with maintaining ongoing “active” CTL immunity.
Studies by Glenny during the first quarter of the century revealed that aluminum compounds could enhance the strength of diphtheria vaccines. This was ostensibly the first of a long history of observations supporting a “depot” theory of immunization, which postulates that antigen slowly leaking into the tissues over an extended time correlates with the antigenic potency of a vaccine. Today, this antigen depot paradigm forms the intellectual backdrop to most adjuvant development programs. In one form or another, depot type adjuvants are intended to prolong the course of antigen delivery, by forming a lesion at the site of injection, or simply by the slow degradability of the adjuvant itself, which mixed with the specific antigen forms a depot at the site of injection. A second function generally attributed to adjuvants are their immunostimulatory effects, which appears to trigger the immune system to respond to the vaccine. However, adjuvants are a double-edged sword. They have inherent toxicities. But it is a feature of these toxicities that achieves a desired immunostimulatory and/or depot effect. Side effects such as tissue damage and granulomatous reaction at the site of injection, fever, and in some cases systemic reactions, such as Reiter's syndrome-like symptoms, uveitis and arthritis, are some of the risks associated with the use of adjuvants. Currently, the only adjuvant approved by the FDA is alum. It is relatively safe but does have side effects such as erythema, subcutaneous nodules, contact hypersensitivity, and granulomatous inflammation. More importantly alum only acts to potentiate a limited number of antigens, and it very predominantly stimulates humoral antibody responses rather than CTL immunity. Thus so far adjuvants have proved to be very ineffective components for vaccines aimed at inducing clinically relevant CTL responses.
Recent attempts to induce CTL responses using dendritic cells or other antigen presenting cells, despite being cumbersome, have shown some promise. New recombinant virus or bacterial systems carrying genes for specific antigen are effective at inducing primary CTL responses. The most effective viruses, for example, that induce strong CTL responses are those which replicate aggressively in the host. Yet because of the risk for serious or lethal complications as a result of infection, recombinant virus used in a cancer vaccine must be only weakly replicative, or be completely replication deficient. This trade-off between virulence and efficacy is at present an intractable problem.
DNA (or polynucleotide) vaccines are also being developed for the purpose of inducing CTL immunity. Once again, the system has intrinsic limitations that preclude its efficacy in inducing long-lasting CTL immunity. The DNA vaccines consist of a plasmid or similar genetic construct for expressing the antigen of interest. Uptake of the plasmid system by cells of the body results in expression of the antigen and induction of CTL. However, once cells expressing the construct have succeeded in inducing CTL, they are themselves targets for eradication by the CTL. The CTL inducing effect is thus again transient. Moreover, the polynucleotide vaccines have thus far suffered from poor efficiency in terms of CTL induction.
With difficulties in achieving strong primary and/or persisting CTL responses, there are a number of clinical trial groups now using repeated injections of cancer vaccines. The use of antigenically complex materials in the vaccine formulation, such as recombinant virus, or the costs associated with repetitive treatment using cultured APC will, however, make such an approach difficult. On the one hand, repetitive immunization with antigenically complex materials drives the immune system to elaborate a humoral antibody, as opposed to a CTL response, while on the other hand, use of a minimal CTL antigen (such as a nonamer peptide) which does not efficiently drive an antibody response, has also failed to induce a CTL response. Attempts to develop adjuvants that enhance the immunostimulatory aspects of minimal CTL antigens have resulted in the production of materials (i.e. adjuvants) that also induce a competing humoral immune response, or, which simply offer little CTL stimulatory effect.
It has also been suggested that certain controlled release technology using microspheres or liposomes with subunit antigens and peptides might be effective to enhance immunogenecity. The combination of sustained release and depot effect is suggested to reduce the amount of antigen needed and eliminate booster shots. However, the preparation of such compositions is difficult and unpredictable, and vaccine formulations based on this technology have not been translated into effective clinical treatments.
As can be seen from the foregoing, there has been little success at developing a CTL vaccine that is both capable of inducing a strong CTL response then sustaining that response over time. The development of a vaccine with these capabilities is essential before effective anti-tumor therapy based on CTL immunity can be contemplated.